Compound Interest Calculator 2026
See exactly how your money grows with compound interest. Enter a principal, interest rate, time period, and optional monthly contributions to get your future value.
Quick Answer
$10,000 invested at 7% for 10 years (monthly compounding) grows to $20,097 — you earn $10,097 in interest on a $10,000 investment. Add $200/month and the balance reaches $55,098. Over 30 years with the same $200/month, you accumulate $257,122 on total contributions of $82,000 — compound interest accounts for $175,122 of that growth.
Investment Details
Compound Interest Results
Future Value
$54,714
after 10 years
Total Contributed
$34,000
Interest Earned
$20,714
The Power of Compounding
Compounding monthly earns you $13,714 more than simple interest over 10 years. Of your $54,714 final balance, you contributed $34,000 and earned $20,714 (61% return on contributions).
Year-by-Year Growth
| Year | Balance | Contributed | Interest |
|---|---|---|---|
| Year 1 | $13,201 | $12,400 | $801 |
| Year 5 | $28,495 | $22,000 | $6,495 |
| Year 10 | $54,714 | $34,000 | $20,714 |
Assumes constant rate and contributions. Does not account for taxes on gains. For illustrative purposes only.
$10,000 Invested — Future Value by Rate & Time
Monthly compounding. No additional contributions.
| Years | 5% Return | 7% Return | 10% Return |
|---|---|---|---|
| 5 years | $12,834 | $14,176 | $16,453 |
| 10 years | $16,470 | $20,097 | $27,070 |
| 20 years | $27,126 | $40,388 | $73,281 |
| 30 years | $44,677 | $81,165 | $198,374 |
| 40 years | $73,584 | $163,113 | $537,006 |
Frequently Asked Questions
How does compound interest work?
Compound interest means you earn interest on your interest — not just on your original principal. In year 1, you earn 7% on $10,000 = $700. In year 2, you earn 7% on $10,700 = $749. By year 10, you're earning 7% on $19,672 = $1,377 — almost double the year-1 interest. Over 30 years at 7%, $10,000 grows to $76,123 — $66,123 of that is pure compound growth, never added from your pocket. This is why starting early is so powerful.
What is the formula for compound interest?
A = P(1 + r/n)^(nt), where: A = future value, P = principal, r = annual interest rate (decimal), n = compounding periods per year, t = time in years. Example: $10,000 at 7% compounded monthly for 10 years: A = $10,000 × (1 + 0.07/12)^(12×10) = $10,000 × 2.0097 = $20,097. Monthly compounding gives slightly more than annual compounding ($19,672), because interest compounds more frequently.
How much does $10,000 grow in 10 years?
At 5% annual return (monthly compounding): $10,000 → $16,470 (+$6,470 interest). At 7%: $10,000 → $20,097 (+$10,097). At 10%: $10,000 → $27,070 (+$17,070). At 12% (S&P 500 historical average): $10,000 → $33,003 (+$23,003). The difference between 7% and 10% over 10 years is $6,973 on the same $10,000 initial investment — underscoring why investment returns matter tremendously over time.
What is the Rule of 72?
The Rule of 72 is a quick mental math shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 7%: 72 ÷ 7 = 10.3 years to double. At 10%: 72 ÷ 10 = 7.2 years. At 6%: 72 ÷ 6 = 12 years. The exact formula (using natural logarithm) gives slightly different numbers, but the Rule of 72 is accurate within a few months for typical rates between 4–15%. It also works in reverse: 72 ÷ 6% inflation = your purchasing power halves in 12 years.
Does compounding frequency make a big difference?
Not as much as the interest rate itself, but it matters. On $10,000 at 7% for 10 years: Annual compounding → $19,672. Monthly compounding → $20,097. Daily compounding → $20,138. The difference between annual and daily compounding is only $466 on $10,000 over 10 years — a 0.47% improvement. However, on $1,000,000 over 30 years, the difference grows to ~$150,000. For savings accounts and CDs that advertise APY (Annual Percentage Yield), the APY already accounts for compounding frequency.
How much should I save monthly to reach $1 million?
Starting from $0 at 7% annual return (monthly compounding): to reach $1 million in 40 years, save $432/month. In 30 years: $912/month. In 20 years: $2,186/month. In 10 years: $5,778/month. The longer your time horizon, the less you need to save monthly because compound growth does more of the heavy lifting. At 10% return: $1 million in 30 years requires only $499/month vs $912/month at 7%.